Integrated Circuits (IC) commonly include multiple semiconductor systems that operate at different voltage supply levels. These semiconductor systems can be laid out on a single semiconductor chip or on different semiconductor chips. A typical semiconductor system has input buffer units, logic core, and output buffers. The logic core in one semiconductor system operates at a different supply voltage level than that in another semiconductor system. To achieve interoperability between semiconductor systems, a high speed Metal Oxide Semiconductor Field Effect Transistor (MOSFET) output driver is used to shift an input signal at one supply voltage level to an output signal at another supply voltage level. In addition, high voltage output buffers are used at the output stage to insure high quality output signals.
Due to the requirements of high speed, low power consumption, high quality signals, and low fabrication costs, semiconductor systems utilize thin gate oxide semiconductor devices. However, thin gate oxide semiconductor components have low breakdown voltage. To protect these components, a reference voltage is introduced to high speed MOSFET output drivers and high voltage cascode output buffers to prevent the gate voltage and drain source voltage from exceeding the breakdown voltage. When an IC device consists of many semiconductor systems operating at different voltage supply levels, it needs different reference voltages to effectively protect the gate oxide of its semiconductor systems.
Some prior art voltage generators generate reference voltages based on either low voltage supply level (LV) or a fraction thereof, on one of more MOSFET threshold voltages above electrical ground, or on high voltage supply level (HV) or a fraction thereof. These prior art generators generate a reference voltage that is optimal for only one high voltage supply level. When systems have the same voltage supply levels (e.g., when HV=LV), these prior art voltage generators provide a reference voltage that is too high. Other prior art voltage generators generate the reference voltage based on one or more MOSFET threshold voltages below high voltage level. These prior art generators are difficult to compensate for temperature and process variations.
Thus, there is a need for a simple voltage generator for high voltage cascode output buffers that can automatically sense both high voltage supply level (HV) and low voltage supply level (LV) in IC devices. In addition, there is a need for a voltage generator that can generate an accurate and stable reference voltage.